In a satellite positioning system such as GPS (Global Positioning System), a satellite positioning receiver receives radio frequency signals from a number of satellites in order to calculate its position. In the case of GPS, radio frequency signals from four different satellites (out of a total of 24 different satellites) are received in order for a GPS receiver to resolve its position in three dimensions.
GPS satellites transmit radio frequency signals according to a Code Division Multiple Access (CDMA) protocol. All of the signals are transmitted in a single frequency band by using orthogonal chipping codes to spread the data across the frequency band. A different chipping code is assigned to each satellite communication channel, enabling a receiver to differentiate between different transmissions.
When a GPS receiver is used initially, or has not been used for a long time, it is said to be making a “cold start”. In this situation, the GPS receiver does not know which of the 24 GPS satellites are visible from its position, what the chipping code or chipping code phase of the communication channels relating to the visible satellites are, or how Doppler shifting has affected the frequency of those communication channels.
Each of the 24 satellites transmits Almanac and Ephemeris data to provide at least some of this “missing information” to GPS receivers. The Almanac data gives the approximate position of all of the satellites in the system and the Ephemeris data enables the precise orbit of each satellite to be calculated. A GPS receiver is able to determine which satellites are in view using the Almanac and Ephemeris data.
However, in low signal conditions, such as if the GPS receiver is located indoors, it may not be possible to receive the Almanac and Ephemeris data, or the data may be received slowly. In this situation, the time to first fix (TTFF) can be as long as ten minutes. This is unacceptable to a user in many circumstances, and may mean that a user is discouraged from using GPS positioning.